Some image forming apparatuses such as copiers, printers, and facsimile machines employ an electrophotographic method as a recording method, and the market for such image forming apparatuses have been rapidly growing.
Conventionally, in such an image forming apparatus employing an electrophotographic method, ion generating devices have been used as a charging device for charging a photoreceptor or the like, as an electricity-removing device for removing electricity from a photoreceptor or the like, and as a transfer device for transferring a toner image onto a transfer receiving material. Currently mainstream examples of the ion generating devices include corotron or scorotron corona charging devices and roller-type contact charging devices.
However, now that image forming apparatuses have been miniaturized, there is a demand for miniaturization of charging devices. In reality, such ion generating devices as corona charging devices and roller-type contact device described above cannot sufficiently meet the demand for miniaturization.
Specifically, a corona charging device is arranged such that a corona wire is so covered with a case as to be approximately 1 cm away from the case. This makes it difficult to miniaturize the charging device. A roller-type contact charging device also requires a space, albeit not as large as a corona charging device does, in which a roller is disposed.
Studied in response to the demand as an ion generating device capable of charging even in a small space is an ion generating device, referred to also as “solid ion generating device, which uses a creeping discharge element that generates ions along with creeping discharge. Such an ion generating device can greatly reduce the amount of ozone to be generated.
FIG. 16 shows an arrangement of a conventional ion generating device for generating ions along with creeping discharge. As shown in FIG. 16, the ion generating device includes: a dielectric material 101; and a creeping discharge element 104, which has a discharge electrode 103 and an inductive electrode 102 disposed so as to face each other via the dielectric material 101.
Each of the discharge electrode 103 and the inductive electrode 102 is formed by forming an electrode film such as a tungsten electrode film on (a surface of) the dielectric material 101 and shaping the electrode film into a ribbon (stripe) with use of a photolithographic technique or the like.
When an alternating voltage is applied between the discharge electrode 103 and the inductive electrode 102, creeping discharge occurs near the discharge electrode 103, so that ions are generated accordingly. In the case of the ribbon discharge electrode 103, discharge occurs in two edge portions that extend in a longitudinal direction of the ribbon discharge electrode 103.
Further, commonly-used examples of the dielectric material 101 include mica paper made by joining pieces of mica on top of each other with resin. Other usable examples of the dielectric material include raw mica, ceramic, and a resin sheet. However, it is very difficult to obtain raw mica having a large amount of space, and such raw mica is expensive. Further, ceramic is expensive, albeit not as expensive as raw mica. Moreover, ceramic is prone to breakages and cracks. This makes it difficult to shape ceramic into a thin plate.
Conventionally, a creeping discharge element in such an ion generating device or the device has been judged to have reached the end of its life when it becomes unable to ensure uniformity in ion generating ability or when its ion generating ability falls short of an allowable lower limit.
In the case of the dielectric material 101 made of mica paper as shown in FIG. 16, there is a minute hole 106 formed in a space between mica flakes 105. As the accumulated amount of discharge time becomes larger, the minute hole 106 grows larger or comes to contain water. That part of the minute hole 106 which has grown larger or contains water deteriorates in insulation resistance, and therefore becomes incapable of discharging. This makes it impossible to ensure uniformity in discharge, thereby causing nonuniformity in ion generating ability.
Further, when a high voltage is applied to the discharge electrode 103, a strong electric field is formed between the discharge electrode 103 and a part of the dielectric material 101 in contact with the discharge electrode 103. Therefore, as the accumulated amount of discharge time becomes larger, the dielectric material 101 suffers from a change in color (burn-in). When the dielectric material 101 suffers from such a change in color, the dielectric material 101 cannot maintain the required ion generating ability.
As described above, conventionally, the creeping discharge element 104 or the ion generating device has been judged to have reached the end of its life at a point of time where the dielectric material 101 deteriorates.
Examples of prior art documents of an ion generating device including a creeping discharge element include Patent Documents 1 and 2. In order to prevent deterioration from being caused by discharge, Patent Documents 1 and 2 teach that a surface of a discharge electrode and a surface of a dielectric material on which the discharge electrode is provided are coated with an inorganic coating agent. Patent Documents 1 and 2 also teach that: the coating agent performs a function of preventing the discharge electrode from being wearing due to discharge and prevents the dielectric material from deteriorating due to discharge, thereby greatly improving the life of the ion generating device.
[Patent Document 1]
Japanese Unexamined Patent Application Publication No. 237368/2002 (Tokukai 2002-237368; published on Aug. 23, 2006)
[Patent Document 2]
Japanese Unexamined Patent Application Publication No. 47642/2006 (Tokukai 2006-47642; published on Feb. 16, 2006)
However, even in the arrangement, described in Patent Documents 1 and 2, in which the discharge electrode and the dielectric material are coated with the inorganic coating agent, it is hard to say that the life of the ion generating device is sufficiently long. A proposal for a technique for further extending the life of a creeping discharge element or an ion generating device has been expected.
Furthermore, such a conventional ion generating device as described in Patent Documents 1 and 2 has problems with cost of manufacturing and running cost.
That is, in the above arrangement, the discharge electrode 103 and the inductive electrode 102 are formed by forming an electrode film on a surface of the dielectric material 101 and patterning the electrode film with use of a photolithographic technique or the like. This inevitably causes a rise in cost of manufacturing.
Further, in the case of the discharge electrode 103 made by patterning the electrode film, creeping discharge occurs mainly in the edge portions as shown in FIG. 16, so that the amount of space of discharge is small. Therefore, the amount of discharge is small with respect to the alternating voltage applied between the discharge electrode 103 and the inductive electrode 102. In order to ensure the amount of discharge, it is necessary to apply a high alternating voltage to some extent. This causes an increase in power consumption, and shortens the life of an ion generating device or a creeping discharge element by accelerating deterioration of the dielectric material 101, thereby causing a rise in running cost.